A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Lithographic apparatus commonly comprise a support frame which is supported by a base via a vibration isolation system, and a projection system arranged to transfer a pattern from a patterning device onto a target portion of a substrate, wherein the projection system comprises a first frame which is spring-supported by the support frame. Spring-supported in this context refers to a resilient support which may alternatively be referred to as spring-mounted, and does not require the actual presence of a spring. Any element having a resilient or spring-like behavior, such as air-mounts may form a spring-support as mentioned in this application.
The mentioned vibration isolation system is used to isolate the support frame from vibrations in the base and may therefore use elements having a resilient or spring-like behavior. Therefore, a mass can be spring-supported by another mass using a vibration isolation system that is arranged in between the two masses, or in other words, the vibration isolation system may form a spring-support. Vibration isolation systems and spring-support are well known to a person skilled in the art and will therefore not be described in more detail.
The first frame may interact with other parts of the projection system such as optical elements, e.g. lens element, mirrors, etc. As the optical elements of the projection system are critical in the imaging performance of the lithographic apparatus, it is desired that the interaction between the first frame and the optical elements minimally disturbs the optical elements. However, it has been found that first frame movements cause disturbances of the optical elements. These movements may originate from movement of the support frame, which in turn may be caused by movement of other masses supported by the support frame, so that movement of e.g. a second frame of the projection system excites movement of the first frame. This effect may be worsened by the fact that resonance frequencies of the first frame can be close to other resonance frequencies of masses connected to the first frame via the support frame. As a result, the imaging performance of the lithographic apparatus is not satisfactory.
Additionally, movement of the first frame may induce deformations of the frame. As the first frame is commonly used to support sensors measuring the optical element positions, these frame deformations may induce position errors in the optical element, again leading to a deteriorated imaging performance of the lithographic apparatus.